Rapid thermal anneal system and method including improved temperature sensing and monitoring

ABSTRACT

A broadband pyrometer is used for sensing temperature of a semiconductor wafer in an RTA system in association with a monochromator to cancel the backside characteristics of the semiconductor wafer. A rapid thermal anneal (RTA) system includes a rapid thermal anneal (RTA) chamber, a heating lamp arranged in the vicinity of the RTA chamber for heating interior to the RTA chamber, a broadband pyrometer disposed in the vicinity of the RTA chamber and directed to measure interior to the RTA chamber, and a grating monochromator connected to the broadband pyrometer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rapid thermal annealing (RTA)semiconductor manufacturing system and method. More specifically, thepresent invention relates to an RTA system and method having improvedtemperature sensing, monitoring and control.

2. Description of the Related Art

Rapid thermal annealing (RTA) is a semiconductor fabrication techniqueusing short-time, high temperature processing to avoid unwanted dopantdiffusion that would otherwise occur at the high processing temperaturesof 900° C. to 1000° C. or greater that are used to dissolve extendeddefects in silicon (Si) and gallium arsenide (GaAs). The duration of anRTA process ranges from seconds to a few minutes so that semiconductorsubstrates are subjected to high temperatures only long enough to attaina desired process effect but not so long that a large degree of dopantdiffusion takes place. RTA is typically performed in specially-designedsystems rather than conventional furnaces or reactors which includesusceptors, wafer boats and reactor walls having a large thermal masswhich prevents performance of rapid thermal cycling. Early RTA processesused lasers as an energy source, allowing a high degree of heating tooccur within fractions of a microsecond without significant thermaldiffusion. Unfortunately, the wafer surfaces had to be scanned by smallspot-size laser beams, causing lateral thermal gradients and waferwarping.

Subsequently, large-area incoherent energy sources were developed toovercome these limitations. These energy sources emit radiant light,which then heats the wafers, allowing very rapid and uniform heating andcooling. RTA systems have been developed in which wafers are thermallyisolated so that radiant, not conductive, heating and coolingpredominates. Temperature uniformity is a primary design considerationin these systems so that thermal gradients, which cause slip andwarpage, are avoided. RTA systems use various heat sources including arclamps, tungsten-halogen lamps, and resistively-heated slotted graphitesheets.

Several difficulties arise in achieving temperature uniformity. First,to raise the temperature of a semiconductor wafer of course requiresheating of the slide carriers and insertion equipment for handling thewafer. The large thermal mass of slide carriers and insertion equipmentextend the process times to at least fifteen to thirty minutes to obtainreproducible results. Significant changes in the doping profile of thewafer can occur over this time, causing difficulty in forming a desiredstructure in the substrate. For example, the precise alignment ofshallow junctions becomes difficult to control when the temperature isnot controllable.

A second problem is that dopants such as arsenic can be lost throughpreferential evaporation effects. In GaAs, arsenic loss is severe withconsiderable deterioration of the semiconductor material unless thesemiconductor is appropriately capped.

Temperature uniformity is typically tested by measuring the emissivityof a semiconductor wafer. Emissivity is defined as the ratio of powerper unit area radiated from a surface to the power radiated by a blackbody at the same temperature when radiation is produced by the thermalexcitation or agitation of atoms or molecules. When a semiconductorwafer is heated, such as occurs in rapid thermal annealing, thetemperature of the wafer is raised and the increase in temperature isdetectable by an optical signal with a characteristic spectrum that isindicative of the wafer temperature. A measurement of emissivityquantifies the characteristic spectrum.

Referring to FIG. 1, an intensity-wavelength plot of the frequencyspectrum response 100 of an infrared pyrometer is shown. In a typicalconventional rapid thermal anneal system, a single fixed-wavelengthpyrometer, for example having a wavelength of 2.7μ, is used to measuretemperature, typically at one or two positions. The frequency spectrumdetected by the infrared pyrometer is neither narrow-band limited orbroad-band limited, having a detection band of a few angstroms ofreceptive wavelength in the vicinity of the infrared region. One problemwhich arises using the infrared pyrometer to detect emissivity is thatonly wavelengths in the relatively limited range of the infraredspectrum are detected.

Thus, the conventional usage of an infrared pyrometer ignores emissivityin other regions of the spectrum, tantamount to an assumption thatemissivity occurs at a constant level across a broad spectrum and thatthe infrared regions is highly representative of the emissivity of thebroad spectrum. However, these assumptions are erroneous.

As a semiconductor wafer is illuminated, the wafer absorbs part of theenergy and reflects part of the energy. The relative amount of energyreflected and absorbed is highly dependent on the type of films on thewafer, which may be highly variable from wafer to wafer. The relativeamount of energy that is reflected and absorbed is highlyposition-dependent in the wafer. The wafer surface generally includesvarious oxides, polysilicon, deposited oxides and the like, generallyhaving variable thicknesses and types. Differences in both the type ofmaterial and the thickness of the material on the semiconductor waferrelate to variability in the absorption and reflectivity of local areasof the wafer, causing variations in emissivity at different regions ofthe semiconductor wafer. For example, absorption of radiant heat by thesemiconductor wafer is related to the free carrier concentration so thatthe heating rate for heavily doped material is more rapid than forsemiconductor wafers with less doping.

Nulls occasionally occur in which substantially no energy is reflectedand thereby detected by the infrared pyrometer. In particular, thevarious types of deposits and deposition thicknesses act as aquarter-wave plate in which energy is absorbed in a material of aparticular type and thickness which is coincident with the effectivewavelength of the pyrometer so that a quarter-wave path difference witha relative phase shift of 90° occurs between ordinary and extraordinarywaves. Thus, substantially all of the energy at the effective wavelengthof the pyrometer is absorbed in the material and very little isreflected. The pyrometer badly misjudges the temperature of the wafer inthese regions, measuring a temperature that is much lower than theactual temperature.

The temperature measurement system is typically used in a feedbackcontrol system which responds to the detected low temperature byincreasing the intensity of the heating lamps or extending the durationof annealing. The increase in RTA processing damages or destroys thesemiconductor wafer in process.

Present day rapid thermal anneal systems typically address the problemsof Emissivity measurement variations and temperature measurementinaccuracies by attempting to construct an ideal RTA chamber,specifically an RTA chamber which is most equivalent to a black bodyradiator so that the only energy absorbing component in the chamber isthe semiconductor wafer. However, even with an ideal RTA chamber,absorption by the semiconductor wafer introduces variability intemperature measurement that may not be compensated.

What is needed is a method and system for monitoring and accuratelycontrolling temperature in a rapid thermal anneal system.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a broadbandpyrometer is used for sensing temperature of a semiconductor wafer in anRTA system in association with a monochromator to cancel the backsidecharacteristics of the semiconductor wafer.

In accordance with one embodiment of the present invention, a rapidthermal anneal (RTA) system includes a rapid thermal anneal (RTA)chamber, a heating lamp arranged in the vicinity of the RTA chamber forheating interior to the RTA chamber, a broadband pyrometer disposed inthe vicinity of the RTA chamber and directed to measure interior to theRTA chamber, and a grating monochromator connected to the broadbandpyrometer.

Several advantages are achieved by the described system and method. Oneadvantage is that temperatures inside an RTA chamber are more accuratelymeasured. Another advantage is that the improved accuracy of temperaturemeasurement allows for improved temperature control in the chamber. Afurther advantage is that usage of a grating monochromator attains ahigh spectral resolution and a high updating rate of several times persecond so that a feedback control system achieves a rapid response.Another advantage is that the grating monochromator distinguishesemissivity at substantially all frequencies thereby supplying detectionof nulls in the spectrum caused by quarter-wave effects and otherdiscontinuities in the temperature measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1, labeled PRIOR ART, shows an intensity-wavelength plot of thefrequency spectrum response of an infrared pyrometer.

FIG. 2 is a schematic block diagram depicting a rapid thermal annealing(RTA) system in accordance with one embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating the operation of a gratingmonochromator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a schematic block diagram illustrates awater-cooled rapid thermal anneal (RTA) system 200 including an RTAchamber 210, a plurality of tungsten halogen lamps 220 for heating asemiconductor wafer 230 within the RTA chamber 210, two broadbandpyrometers 240 for monitoring the heating of the semiconductor wafer230, and one or more grating monochromators 250 respectively associatedwith the broadband pyrometers 240. Two examples of broadband pyrometersare pyrometers within infrared spectrometers manufactured by NicoletInstrument Corp, Madison, Wis., and manufactured by The Perkin-ElmerCorporation, Norwalk, Conn. Point supports 212 hold the semiconductorwafer 230 to alleviate thermal mass effects. The temperature ismonitored by measuring emissivity with the pyrometers 240 and usingfeedback control of the emissivity measurement to control heating by thelamps 220. The RTA system 200 further includes a control circuit 260connected to the broadband pyrometers 240, the grating monochromators250 and the tungsten halogen lamps 220.

The control circuit 260 is connected to the broadband pyrometers 240 toreceive signals indicative of emissivity and, therefore, temperaturewithin the RTA chamber 200. Circuits with connections to a measurementdevice such as a pyrometer or thermocouple for measuring or derivingtemperature are well known. The control circuit 260, which includesswitches and an adjuster such as a potentiometer or pulse widthmodulation. system for controlling the time and intensity of heating.Circuits with connections to heating elements for controlling the timeand intensity of heating are well known. The control circuit 260 isconnected to the grating monochromators 250 for adjusting the wavelengthof radiation passed to the broadband pyrometers 240.

Circuits with connections to a grating monochromator or spectrometer foradjusting the radiation wavelengths passed by a filter are well known.

RTA chamber 210 is a heating chamber supplying a controlled environmentfor the semiconductor wafer 230 and for supplying energy from the lamps220 to the semiconductor wafer 230. The RTA chamber 210 forms anair-tight structure so that heating is supplied under conditions ofinert atmospheres of argon (Ar) and nitrogen (N₂) or a vacuum. In someRTA systems, oxygen and ammonia is supplied for growth of oxide (SiO₂)and nitride (Si₃ N₄) into an RTA chamber. Nitrogen source gases such asNO and N₂ O can be used to implant nitride into oxide films.

A plurality of lamps 220, such as arc lamps, tungsten-halogen lamps andthe like, are arranged about the RTA chamber 210. In the illustrativeRTA system 200, the lamps 220 are arranged in a suitable linear array222. In other embodiments, the lamps 220 are arranged in other suitableformations including, for example, an hexagonal array of lamps. Thelamps 220 have a controlled intensity. In some embodiments, all lampsare controlled commonly. In other embodiments, each lamp is individuallycontrolled. Switched incoherent heat sources are typically used althoughcoherent heat sources may also be used.

The illustrative RTA system 200 includes two broadband pyrometers 240for monitoring wafer temperature, representing a suitable balance oft hecost of a broadband pyrometer and the benefits attained by improvedsensing. Other RTA system embodiments may include one pyrometer or threeto five pyrometers. Essentially any number of pyrometers may beemployed. Usage of multiple pyrometers allows usage of a singlepyrometer to monitor the center of the backside of the semiconductorwafer 230 and an additional array of pyrometers to measure temperatureas a function of angular position. The measurement of multiple positionson a semiconductor wafer 230 is used to establish temperature uniformityvia a temperature feedback control system.

In the illustrative embodiment, two broadband pyrometers 240 are usedfor monitoring uniformity of heating in a semiconductor wafer 230 duringrapid thermal annealing by measuring emissivity of the semiconductorwafer 230. The broadband pyrometer 240 is sensitive to a wide portion ofthe infrared spectrum. One example of a suitable pyrometer 240 is atotal radiation pyrometer which functions on the basis of theStefan-Boltzmann law E=σT⁴ relating the radiant flux per unit areaemitted by a black body to the temperature. E is the radiant exitante, σis the Stefan-Boltzmann constant and T is the temperature. Also, theStefan-Boltzmann constant σ=2π⁵ k⁴ /15h³ c² where k is the Boltzmannconstant, c is the speed of light in a vacuum and h is the Planckconstant.

Thus, the broadband pyrometer 240 is sensitive to a wide portion of theinfrared spectrum but does not discriminate between absorption andreflection at different wavelengths. A grating monochromator 250functions as an input filter to each broadband pyrometer 240 to sharplynarrow the band of frequencies applied to the broadband pyrometer 240 toa monochromatic band, approximating a single frequency. Each gratingmonochromator 150 is adjusted over time to vary the band of wavelengthsapplied to the broadband pyrometer 240 for accurately sampling a narrowportion of the spectrum for several frequency ranges to generate datarelating intensity to wavelength over a wide range of wavelengths.

The grating monochromators 250 are used to control the measured band offrequencies, rather than another band adjustment technique, because thegrating monochromator 250 achieves a very fast scan time. Using agrating monochromator 250, a large number of monochromatic bands, forexample approximately 2-3 decades of frequency per second, may bemonitored, covering the entire broadband spectrum of the pyrometer 240in less than one second. Usage of the grating monochromators 250advantageously achieves a high spectral resolution and a high updatingrate of several times per second, attaining a rapid response in atemperature sensed feedback control system. By scanning the entirebroadband spectrum of the broadband pyrometer 240, the gratingmonochromator 250 distinguishes the emissivity at substantially allfrequencies supplying detection of nulls in the spectrum caused byquarter-wave effects and other discontinuities in the temperaturemeasurement.

Each broadband pyrometer 240 measures emissivity through a gratingmonochromator 250 which filters the radiation from the semiconductorwafer 230 into monochromatic radiation, which is electromagneticradiation of a single frequency or, more accurately, radiation within anarrow range of frequencies. The grating monochromator 250 is controlledto sample intensity for multiple narrow ranges of frequency. The gratingmonochromator 250 functions as an interference filter for examining anemission spectrum with radiation from the semiconductor wafer 230passing through a collimator (not shown) which produces a parallel beamof radiation that is deviated by an angular deviation and dispersed by adiffraction grating (not shown). The angular deviation depends onwavelengths of the radiation. The refracted or diffracted radiation isobserved or recorded to allow measurement of the angular deviation.

The grating monochromator 250 controls the monitoring wavelength so thatthe semiconductor wafer 230 is scanned over a plurality of frequenciesand scanned across the wafer surface to determine the location ofintensity nulls that are indicative of point absorbencies.

Referring to FIG. 3, a schematic diagram illustrating the operation of agrating monochromator 250 shows radiation entering at an entry 310,reflected by a concave grating 312 through an exit slit 314 onto thebroadband pyrometer 240. In this embodiment, both the entry slit 310 andthe exit slit 314 are held at a constant position and the grating 212 isrotated through a range of angles. Furthermore, in this embodiment acollimator is optional because the grating 312 is ruled on a concavemirror which focuses the radiation. At a particular angle of the grating312, radiation of a specific wavelength is focused onto the exit slit314. The angle of the grating 312 is scanned so that a plurality ofmonochromatic measurements are taken.

While the invention has been described with reference to variousembodiments, it will be understood that these embodiments areillustrative and that the scope of the invention is not limited to them.Many variations, modifications, additions and improvements of theembodiments described are possible. For example one broadband pyrometermay be used in an RTA system or multiple broadband pyrometers may beused. Similarly, a single grating monochromator may be supplied for eachbroadband pyrometer, a single grating monochromator may be supplied formultiple broadband pyrometers or multiple grating monochromators may besupplied for a single broadband pyrometer.

What is claimed is:
 1. A method of sensing temperature in a rapidthermal anneal (RTA) system comprising the steps of:heating the interiorof a rapid thermal anneal (RTA) chamber; measuring radiation interior tothe RTA chamber using a broadband pyrometer directed to said interior ofthe RTA chamber; and filtering radiation directed to the broadbandpyrometer to a plurality of narrow wavelength bands using a gratingmonochromator coupled to the broadband pyrometer.
 2. A method accordingto claim 1 further comprising the steps of:scanning a plurality offiltering wavelengths of the grating monochromator; and measuring atemperature interior to the RTA chamber at a plurality of radiationwavelengths corresponding to the plurality of filtering wavelengths ofthe grating monochromator.
 3. A method according to claim 2 wherein thestep of scanning a plurality of filtering wavelengths of the gratingmonochromator further comprises the step of:controlling a monitoringwavelength; scanning a semiconductor wafer interior to the RTA chamberover a plurality of monitoring wavelengths; scanning across thesemiconductor wafer surface; and determining the location of intensitynulls indicative of point absorbencies.
 4. A method according to claim 2wherein the step of scanning a plurality of filtering wavelengths of thegrating monochromator further comprises the step of:scanning asemiconductor wafer interior to the RTA chamber over a plurality offrequencies; distinguishing emissivity at substantially all frequenciesgenerating detection nulls in an emissivity spectrum caused byquarter-wave effects and temperature measurement discontinuities.
 5. Amethod according to claim 2 wherein the step of scanning a plurality offiltering wavelengths of the grating monochromator further comprises thestep of:monitoring monochromatic bands covering the entire broadbandspectrum of the broadband pyrometer in less than one second.
 6. A methodaccording to claim 1 further comprising the steps of:adjusting theintensity and time duration of heating of said interior of the PTAchamber in response to the measured radiation in a temperature sensedfeedback control system.
 7. A method of controlling a rapid thermalanneal (RTA) system comprising the steps of:heating the interior of arapid thermal anneal RTA chamber; measuring radiation interior to theRTA chamber using a broadband pyrometer directed to said interior ofthe, RTA chamber; filtering radiation directed to the broadbandpyrometer to a plurality of narrow wavelength bands using a gratingmonochromator coupled to the broadband pyrometer; detecting radiationintensity for the plurality of narrow wavelength bands; and adjustingthe intensity and duration of heating in response as a function of thedetected radiation intensity.
 8. A method according to claim 7 furthercomprising the steps of:adjusting the intensity and time duration ofheating of said interior of the RTA chamber in response to the measuredradiation in a temperature sensed feedback control system.
 9. A methodaccording to claim 7 further comprising the steps of:scanning aplurality of filtering wavelengths of the grating monochromator; andmeasuring a temperature interior to the RTA chamber at a plurality ofradiation wavelengths corresponding to the plurality of filteringwavelengths of the grating monochromator.
 10. A method according toclaim 9 wherein the step of scanning a plurality of filteringwavelengths of the grating monochromator further comprises the stepof:controlling a monitoring wavelength; scanning a semiconductor waferinterior to the RTA chamber over a plurality of monitoring wavelengths;scanning across the semiconductor wafer surface; and determining thelocation of intensity nulls indicative of point absorbencies.
 11. Amethod according to claim 9 wherein the step of scanning a plurality offiltering wavelengths of the grating monochromator further comprises thestep of:scanning a semiconductor wafer interior to the RTA chamber overa plurality of frequencies; distinguishing emissivity at substantiallyall frequencies generating detection nulls in an emissivity spectrumcaused by quarter-wave effects and temperature measurementdiscontinuities.
 12. A method according to claim 9 wherein the step ofscanning a plurality of filtering wavelengths of the gratingmonochromator further comprises the step of:monitoring monochromaticbands covering the entire broadband spectrum of the broadband pyrometerin less than one second.